In Optics Letters, Vol. 4, No. 2, February 1979, pp. 58-59, R. Normandin (the present inventor) et al, reported the nonlinear mixing of oppositely propagating guided waves. The resultant field was coupled to radiation modes and propagated in a direction perpendicular to the waveguide surface, in the case of equal frequency fundamentals. In subsequent articles, its application to picosecond signal processing, the creation of all optical transient digitizers and spectrometers demonstrated the potential usefulness of this work. (See Appl. Phys. Lett. 36(4), 15 Feb. 1980, pp. 253-255 by R. Normandin et al; 40(9),1982, pp. 759-761 by R. Normandin et al, and "Integrated Optical Circuits and Components" edited by L. D. Hutcheson, Dekker Inc. New York, U.S.A., Chapter 9, by G. I. Stegeman et al.) The overlap of the two oppositely propagating fields will give rise to a nonlinear polarization source at the sum frequency. In bulk media such a process is nonradiative due to the simultaneous requirement of energy and momentum conservation in all directions. This is not the case in a waveguide geometry.
Unfortunately since the waves do not grow with distance (no phase matching) the resultant fields are much weaker than that obtained in a traditional second harmonic generation device. Therefore, this nonlinear interaction has remained largely a laboratory curiosity. However, the applicant's copending application Ser. No. 07/545,497 filed on June 26, 1990, now U.S. Pat. No. 5,051,617, describes successful ways of increasing this interaction by factor of 10.sup.7 and obtaining efficient conversion in the visible region. As mentioned in the copending application, with presently available diode laser sources, most of the visible spectrum can be reached. Ultrafast subpicosecond samplers and monolithic high resolution spectrometers are possible in the context of fiber optics communication systems and optoelectronic integrated circuitry.
Further research has been conducted on the multilayer structures. Various parameters have been considered, e.g. refractive index, thickness and location of layers etc. in order to improve the interaction bandwidth so that wider application fields can be envisaged.